Thermal diffusion method



Oct. 23, 1956 E. N. MARSH 2,767,850

a THERMAL DIFFUSION METHOD Filed Deo. 18, 1955 HIS HTTORNYS United States Patent Office 2,767,859 Patented Oct. 23, 1956 2,767,850 THERMAL DIFFUSION METHOD Edward N. Marsh, Shaker Heights, Ohio, assignor to The Standard Oil Company, Cleveland, Ohio, a corporation of Ohio Application December 18, 1953, Serial No. 398,935 5 Claims. (Cl. Zul-52.5)

This invention relates to an improved continuous liquid thermal diffusion method and apparatus and particularly to improvements in the flow patterns of thermal diffusion methods and apparatus of the types described in U. S. Patents 2,541,070 and 2,541,071 to Jones and Hughes and in U. S. Patent 2,521,112 to Beams.

The Jones and Hughes patents referred to above describe a continuous method for separating, by thermal diffusion, two fractions containing dissimilar materials that are normally liquid under the conditions of separation and which are included in a mixture that is normally liquid under the conditions of separation. The term dissimilar components, as used in the .T ones and Hughes patents and in the description of the present invention, is intended to refer to two or more -components in or of a liquid or liquifiable mixture. These components may be dissolved in a common solvent, they may be liquid components of a mixture or one of the components may be a solvent and other the solute.

The method of the Jones and Hughes patents in essence involves continuously introducing a liquid mixture into apparatus comprising a substantially vertical and narrow separation chamber at a point intermediate the upper and lower ends thereof. The chamber is formed by smooth side walls, the opposed faces of which are closely and uniformly spaced apart. One of these walls is relatively heated and the other is relatively cooled to impose a temperature gradient across the separation chamber or slit formed by these walls and consequently also across the thin film or layer of liquid mixture in the chamber. As a result of the relative heating of the liquid immediately adjacent the hot wall, a term used in a relative sense, and the relative cooling of the liquid immediately adjacent the cold wall, a term likewise used in a relative sense, two phenomena take place. One of these is the well-known phenomenon of thermal circulation or convection which resolves the liquid in the chamber into two countercurrently mting fractions, one ascending adjacent the hot wall and the other descending adjacent the cold wall. The other phenomenon, not so wellknown, is that of thermal diffusion whereby different amounts of energy of translation are imparted to dissimilar molecules in the mixture with the result that one type of molecule tends to accumulate adjacent the hot wall and the other tends to accumulate adjacent the cold wall. Consequently, the ascending fraction in the chamber becomes progressively more concentrated in one type of dissimilar molecule and the descending fraction becomes progressively more concentrated in the other type or impoverished in the first type of dissimilar molecules. Withdrawal of a portion of the ascending fraction from the top of the separation chamber and withdrawal of a portion of the descending fraction from the bottom of the separation chamber results in achieving significant separations of dissimilar components in the liquid mixture subjected to thermal diffusion.

In the Beams patent, it is likewise proposed to introduce the liquid mixture continuously into va thermal vdiffusion chamber at a point intermediate its ends. The separation chamber, however, is in a horizontal position and the upper wall is relatively heated, whereas the lower wall is relatively cooled. To supply a means for circulating the liquid in the chamber countercurrently, since this cannot be achieved thermally in a horizontal chamber, the patentee proposed to utilize an endless tape moving countercurrently through the chamber and thereby in effect dragging a fraction accumulating adjacent the upper hot wall toward one extremity of the chamber and a fraction accumulating adjacent the lower cold wall toward the opposite end of the chamber for separate removal.

It has now been determined that the methods and apparatus described in these patents have an inherent disadvantage in that the portion of a given fraction which reaches the end of a separation chamber but is not withdrawn is forced, by displacement, to reverse its direction of movement along the opposite chamber-forming wall surface. This displacement of the non-withdrawn portion of the fraction constitutes an inherent source of inefficiency in that further energy must be expended to move the component concentrated in that portion toward the first chamber-forming surface. Thus, for example, when a fraction that becomes progressively more concentrated in a given component as it descends along the cold wall of a vertical separation chamber is only partially withdrawn upon reaching the lower end of the chamber, the balance of the fraction must turn around and begin an ascent along the hot wall. In order to drive said given component in this turned around and ascending fraction back to the descending fraction adjacent the cold wall, a further amount of heat is required to impart the necessary additional kinetic energy of translation to the molecules thereof. Substantially the same is true when only part of the ascending stream adjacent the hot wall is withdrawn at the top of a vertical separation chamber or when only part of a fraction accumulating adjacent the hot or cold wall of a horizontal separation chamber of the Beams type is withdrawn at one end or the other.

The present invention is directed specifically to the problem of avoiding this inherent ineiiiciency in hitherto proposed vertical and horizontal countercurrent flow thermal diffusion methods.

In accordance with the method of the invention, the liquid mixture to be subjected to separation into liquid fractions containing dissimilar components is confined in a thermal diffusion separation chamber, subjected to a temperature gradient to separate the liquid mixture into two countercurrently moving fractions, one adjacent each of the opposed chamber-forming walls. At least one of the fractions is continuously withdrawn substantially completely from the outer end of the chamber toward which the fraction in question moves. Fresh liquid mixture is continuously introduced into the chamber at a location intermediate the outer ends and additional liquid mixture, which may be identical with the fresh liquid mixture or may contain the same dissimilar components in different concentrations, is introduced as auxiliary feed into the end or ends of the chamber from which a fraction or fractions is or are substantially completely withdrawn without substantially diluting said fraction or fractions.

The apparatus of the present invention in essence comprises at least two smooth, substantially parallel walls having surfaces closely spaced apart to form at least one thermal diffusion separation chamber, means for relatively heating and cooling the opposed wall surfaces, an inlet for the chamber remote from at least one of the ends of the chamber, an outlet at each end of the chamber, an auxiliary inlet at at least one of the ends of the chamber, and means for avoiding substantial intermixing,

at one or both ends of the chamber, between liquid introduced through the auxiliary inlet or inlets and the fraction or fractions withdrawn through the outlet or outlets of the same end or ends of the chamber.

it is apparent from the foregoing that a number of flow patterns and structures are contemplated in the meth-od and apparatus, respectively of the present invention. Thus, for example, the countercurrent movement o the fractions separated within the chamber by thermal ditusion, and the chamber itself, may be vertical or horizontal. When the flow pattern and the apparatus are verti ai, thermal circulation is ordinarily vlent to impart the desired countercurrent movement to the dissimilar fractions. It the ilow pattern and apparatus are horizontal, in which case thermal circulation is not a tact-or, the desired countercurrent movement of the sepa- "ted fractions must be promoted by other means such c moving tape means proposed in the Beams patent. e the liow pattern is vertical, the fresh liquid is ed at a location intermediate the upper and lower ends oi the chamber, preferably at about the midpoint thereof, and (n) substantially the entire ascending fracbe withdrawn from the top, in which case andA t i liquid mixture will be introduced at the upper unl of the chamber' through an auxiliary inlet without "itially diluting the ascending fraction, (b) substan- `y the entire descending fraction may be withdrawn h end of the chamber, in which case an additional amount of liquid mixture will be introduced through an auxiliary inlet at the lower end of the chamber' without substantially diluting the descending fraction, or (c) cath the ascending and descending fractions will be sul: ntially completely withdrawn at the upper and ends, respectively, of the column, in which case s:cual liquid mixture will be introduced through auxry inlets at the upper and lower ends of the column without substantially diluting the ascending and descending fractions, respectively. While the various llow pat s within the scope of the invention have been ded with reference to a vertical separation chamber, it is to he understood of course that the same iiow patter s are possible in a horizontal separation chamber in n the upper wall is the hot wall and the lower wall the cold wall.

The apparatus of the invention may be modilied depending upon the particular tlow pattern desired` Thus, t... c .an1ple, where it is desired to withdraw substanf tially the entire ascending fraction in a vertical separation chamber and to introduce additional liquid mixture through an auxiliary inlet at the upper end of the chamber, means such as a baille or the like are provided at the upper end of the chamber between the outlet and the i et to avoid substantial dilution of the ascending ir: tion by the entry of the liquid mixture. Similarly, il` substantially the entire descending fraction is to be idrawn at the lower end of the chamber, such means are provided between the outlet and the auxiliary inlet f the lower end. if both the ascending and descending vfactions are to be substantially completely withdrawn, uch barrier means are provided at both ends of the "li-amber. Similar modifications are contemplated for thc various embodiments wherein the separation chamber 'i in the horizontal position.

in the preferred embodiment of the invention, liquid mixture is confined in a substantially vertical separation chamber' because of the inherently greater simplicity of structures that do not require moving parts. The width ot the separation chamber, i. e., the spacing between the hot and cold walls, should be no greater than about 0.15 inch and preferably `uetween about 0.02 and 0.06 inch.

The primary advantage of the method and apparatus et the invention is in avoiding nullication of a substautial amount of the separation that takes place within a thermal diffusion chamber by displacing components concentrated adjacent the hot wall toward the cold wall and vice versa, and thereby increasing the inherent elliany corresponding increase in the amount of heat i i. the

line 2 2 of Figure l;

-ow pattern produced in the apparatus form shown tres l and 2;

4 to 6, inclusive. schematically illustrate other E g c 7 is a schematic illustration of a cascade type llow pattern involving the use of a number of thermal diffusion separation chambers, the flow pattern of each As shown in Figures l and 2, a typical form of the apparatus of the invention comprises two wall members f and il having smooth and substantially parallel surfaces and 14 such a gas 16 to form a thermal diffusion separar The wall member ill is provided with a coil i.; tor passage of a heating medium, an inlet Ztl and associated conduit 21, an auxiliary inlet ociated conduit 24, and an outlet 2o and associated conduit 27. The wall member 11 is provided with a coil 29 for passage of a cooling medium therethrough or conduit 31, and an outlet 32 and associated conduit 34. Barriers or the like 36 and 37 are provided at the upper and lower ends of the chamber l'l between the opposed and 22 and 32, on the other.

in Figures 3-7, the letters C and rl designate he: and cold walls, respectively; lc and Pn indicate the cold stand for feed of fresh liquid mixture and auxiliary feed of liquid mixture, respectively.

ln operation, fresh liquid mixture is continuously inand inlet 20 with the result that the central portion of the chamber i7 constitutes a mixing zone in which the proportions of the dissimilar components are substanliquid mixture. As the liquid moves away from the central mixinfy zone, it becomes resolved, by virtue of the temperature gradient imposed across it from the hot wall progressively more concentrated in a first dissimilar component of the liquid mixture and ascending adjacent the hot wall, and the other fraction being progressively more gressively less concentrated in the first dissimilar c0m ponent, and descending along the cold wall. When the ascending fraction reaches the upper end of the chamoutlet 26 and associated conduit 27. Similarly, when the fract1on descending along the cold wall reaches the lower end of the chamber, it is substantially completely withditional liquid mixture is introduced into the chamber 17 at the upper end by way of conduit 31 and auxiliary inlet 30 and at the lower end by way of conduit 24 and in avoiding substantial dilution of the ascending fraction withdrawn through outlet 26 by the liquid mixture introduced through auxlliary inlet 30 and the barrier or divider of the descending fraction withdrawn by way of outlet 32 by the additional liquid mixture introduced through the auxiliary inlet 22.

Figure 3 is a ldow diagram schematically illustrating iiow panerns within the scope of the invention; and

which corresponds to that illustrated in Figure 4.

closely spaced by any suitable means tien chamhe 22 and equivalent cooling means, and an inlet 30 and associated outlets and auxiliary inlets 26 and 30, on the one hand,

and 'not wail fractions, respectively; and E and Al:

troducetl into the chamber 3X7 by way of conduit 2l tially the same as the proportions thereof in the fresh to the cold wall, into two fractions, one fraction being concentrated in a second dissimilar component, or prix ber 17, it is substantially completely withdrawn through drawn through outlet 32 and associated conduit 34. Ad-

auxiliary inlet 22. The barrier or divider 36 is effective 37 is similarly effective in avoiding substantial dilution By removing the lower barrier or divider 37 and utilizing either or both the ports 22 and 32 as outlets for a portion of the descending fraction, the apparatus schematically illustrated in Figure 1 vmay readily be converted to produce the fiow pattern schematically illustrated in Figure 4. In this embodiment, the fresh liquid mixture is introduced into the chamber 17 by way of inlet 20, substantially the entire ascending fraction is continuously removed by way of outlet 26, additional liquid mixture is introduced at the upper end by way of auxiliary inlet 3f! without substantial intermixing of the ascending fraction due to the action of barrier or divider 36, and a portion of the descending fraction is continuously withdrawn at the lower end of the chamber through ports 32 and 22 or both.

By eliminating the barrier or divider 36 of the apparatus illustrated schematically in Figures 1 and 2 and utilizing one or both the ports 26 and 30 as outlets for a portion of the ascending fraction, the apparatus may readily be converted to produce the flow pattern shown in Figure 5. In this modification, the fresh liquid mixture is introduced by way of inlet 20, a portion of the ascending fraction is withdrawn at the upper end of the chamber 17 by way of ports 26 or 30 or both, substantially the entire descending fraction is Withdrawn a't the lower end of the chamber by Way of outlet 32, and additional liquid mixture is introduced at the lower end of the chamber by way of auxiliary inlet 22, substantial dilution of the descending fraction by the auxiliary feed being avoided by the action of barrier or divider 37.

Figure 6 illustrates schematically a flow pattern corresponding to that illustrated in Figure 4, turned on it side. This fiow pattern is considered the equivalent, in a horizontal separation chamber of the Beams type, of the vertical fiow pattern illustrated in Figure 4 and ymay readily be obtained by modifying the Beams type apparatus in a manner similar to that described with reference to Figures 1 and 2. It is to be understood, of course, that flow patterns corresponding to those shown in Figures 3 and 5 are likewise possible in the Beams type apparatus and come within the scope of the present invention.

While the liquid mixture introduced into the separation chamber by way of one or more auxiliary inlets may be identical with the fresh liquid mixture, it need not necessarily be identical in both composition and relative proportion of the dissimilar components. A flow pattern in which this is illustrated is shown in Figure 7 in which the letters, A, B, C and D designate four separation chambers similar to that illustrated in Figures l and 2, but modified as described to the flow pattern of Figure 4. The letters C and H are utilized to indicate cold and hot walls of the separation chambers, respectively; F indicates the feed of fresh liquid to be subjected to separation by thermal diffusion; Ph designates the hot wall fraction; and Pc designates the cold wall fraction. In thermal diffusion chamber A the primary feed of liquid mixture at the center of the chamber and the auxiliary feed at the upper end are identical. In chambers B, C and D, however, the auxiliary feed in each instance corresponds to the portion of the cold wall fraction removed from the lower end of the previous chamber in series. The cascade type of flow pattern illustrated in Figure 7 is particularly advantageous in instances in which the primary interest is in concentrating a component that tends to accumulate adjacent the cold wall of a thermal diffusion column. It is to be understood, of course, that if the primary interest is in concentrating a component that tends to accumulate adjacent the hot wall, a similar cascading type of flow pattern, wherein the flow pattern of each individual separation chamber is similar to that of Figure 5, will be more applicable.

A determination was made, for comparative purposes, of the results obtainable by subjecting a 50:50 mixture of cetane and monomethyl naphthalene to thermal diffusion, utilizing, on the one hand, the vertical flow pattern ofyFigure 4, and on the other hand, the continuous vertical countercurrent fiow method heretofore proposed, i. e., with the auxiliary feed at the upper end of the column eliminated. All other conditions, such as the width, length and breadth of the chamber, and the temperature gradient, were the same. In run No. 2, the auxiliary feed was the same as the primary feed. The results of these comparisons are tabulated immediately below:

While the degree of separation of the cetane fraction removed from the upper end of the chamber is slightly reduced by the use of an auxiliary feed, it is important to note that the volume of the fraction withdrawn from the upper end of the chamber is doubled while the total feed rate is increased by a factor of only 1.5. This means that for the flow pattern of Figure 4, essentially the same concentration of components, in the fraction Withdrawn from the upper end of the chamber, can be obtained with half the heat consumption. Since the cost of heat is the greaest operating cost in a thermal diffusion operation, the reduction in heat consumption represents a significant reduction in direct operating costs. Furthermore, it appears that the separating efficiency of run No. 2, illustrated schematically in the flow pattern of Figure 4, is 31% higher than that of the flow pattern in run No. 1, with which it was compared, the separating efficiency being defined as the ratio of the product of the volume of the hot wall fraction and the concentration of the desired component therein to the product of the volume of the feed and the concentration of the desired component in the feed.

It is to be expected that various modifications will readily occur to those skilled in the art upon reading this description. All such modifications are intended to come within the scope of the invention as defined in the accompanying claims.

I claim:

1. A method for continuously separating, by thermal diffusion, two fractions containing dissimilar components that are normally liquid under the conditions of separation and which are included in a mixture normally liquid under the conditions of separation, which comprises confining the liquid mixture in a uniformly narrow separation chamber having substantially smooth walls and two outer ends; imposing a temperature gradient across the liquid by maintaining one Wall at a temperature higher than the temperature of the other wall, thereby concentrating one of said dissimilar components in the mixture in a first fraction adjacent one wall and another of said dissimilar components in a second fraction adjacent the other wall; moving said first and second fractions countercurrently and endwise within the chamber toward the outer ends thereof; continuously withdrawing the first fraction from one outer end of the chamber; continuously withdrawing the second fraction from the other outer end of the chamber; continuously in-troducing a first liquid mixture into one of the outer ends of the chamber without substantially diluting the fraction advancing toward said one of said outer ends; and continuously introducing a second liquid mixture into the chamber at a location intermediate said outer ends.

2. A method for continuously separating, by thermal diffusion, two fractions containing dissimilar components that are normally liquid under the conditions of separation and which are included in a mixture normally liquid under the conditions of separation, which comprises confining the liquid mixture in a uniformly narrow and substantially vertical separation chamber having substantially vertical and smooth walls and an upper and a lower end; imposing a temperature gradient across the liquid by maintaining one wall at a temperature higher than the temperature of the other wall, thereby concentrating one of said dissimilar components in the mixture in a first fraction ascending adjacent the wall maintained at the higher temperature and another of said dissimilar components in a second fraction descending adjacent the otherl wall; continuously withdrawing the first fraction from the upper end of the chamber; continuously withdrawing the second fraction from the lower end of the chamber; continuously introducing a first liquid mixture into one of the upper and lower ends of the chamber without substantially diluting the fraction withdrawn from said one of said upper and lower ends; and continuously introducing a second liquid mixture into the chamber at a location intermediate said upper and lower ends.

3. A method for continuously separating, by thermal diffusion, two fractions containing dissimilar components that are normally liquid under the conditions of separation and which are included in a mixture normally liquid under the conditions of separation, which comprises confining the liquid mixture in a uniformly narrow and substantially vertical separation chamber having substantially vertical and smooth walls and an upper and a lower end; imposing a temperature gradient across the liquid by maintaining one wall at a temperature higher than the temperature of the other wall, thereby concentrating one of said dissimilar components in the mixture in a first fraction ascending adjacent the wall maintained at the higher temperature and another of said dissimilar components in a second fraction descending adjacent the other wall; continuously withdrawing the first fraction from the upper end of the chamber; continuously withdrawing the second fraction from the lower end of the chamber; continuously introducing a first liquid mixture into the upper end of the chamber without substantially diluting the first fraction; and continuously introducing a second liquid mixture into the chamber at a location intermediate the upper and lower ends.

4. A method for continuously separating, by thermal diffusion, two fractions containing dissimilar components that are normally liquid under the conditions of separation and which are included in a mixture normally liquid under the conditions of separation, which comprises confining the liquid mixture in a uniformly narrow and substantially vertical separation chamber having substantially vertical and smooth walls and an upper and a lower end; imposing a temperature gradient across the liquid by maintaining one wall at a temperature higher than the temperature of the other wall, thereby concentrating one of said dissimilar components in the mixture in a first fraction ascending adjacent the wall maintained at the higher temperature and another of said dissimilar components in a second fraction descending adjacent the other wall; continuously withdrawing the first fraction from the upper end of the chamber; continuously withdrawing the second fraction from the lower end of the chamber; continuously introducing a first liquid mixture into theV lower end of the chamber without substantially diluting the second fraction; and continuously introducing a second liquid mixture into the chamber at a location intermediate the upper and lower ends.

5. A method for continuously separating, by thermal diffusion, two fractions containing dissimilar components that are normally liquid under the conditions of separation and which are included in a mixture normally liquid under the conditions of separation, which comprises confining the liquid mixture in a uniformly narrow and substantially vertical separation chamber having substantially vertical and smooth walls and an upper and a lower end; imposing a temperature gradient across the liquid by maintaining one wall at a temperature higher than the temperature of the other wall, thereby concentrating one of said dissimilar components in the mixture in a first fraction ascending adjacent the wall maintained at the higher temperature and another of said dissimilar com` ponente in a second fraction descending adjacent the other wall; continuously withdrawing the first fraction from the upper end of the chamber; continuously withdrawing the second fraction from the lower end of the chamber; continuously intorducing a first liquid mixture into the upper and lower ends of the chamber without substantially diluting the first and second fractions; and continuously introducing a second liquid mixture into the chamber at a location intermediate the upper and lower ends.

References Cited in the le of this patent UNITED STATES PATENTS 2,521,112 Beams Sept. S, 1950 2,541,069 Jones et al. Feb. 13, 1951 2,541,070 Jones et al. Feb. 13, 1951 2,541,071 Jones et al. Feb. 13, 1951 2,723,758 Lupfer et al. Nov. l5, 1955 

